1. Technical Field
The present invention relates to a process for drilling into a subterranean formation for the recovery of hydrocarbons, and more particularly to a drilling process wherein a polymer enhanced foam is employed as the drilling fluid.
2. Description of Related Art
A drilling fluid is a specially designed fluid that is circulated through a wellbore as the wellbore is being drilled to facilitate the drilling operation. The circulation path of the drilling fluid typically extends from the surface wellhead down through the drill pipe string to the drilling face and back up through the annular space between the drill pipe string and wellbore face to the surface wellhead. The drilling fluid performs a number of functions as it circulates through the wellbore including cooling and lubricating the drill bit, removing drill cuttings from the wellbore, aiding in support of the drill pipe and drill bit, and providing a hydrostatic head to maintain the integrity of the wellbore walls and prevent well blowouts. The drilling fluid also desirably prevents sloughing and wellbore cave ins when drilling through water sensitive formations.
There are a number of conventional drilling fluids including compositions termed "drilling muds". Drilling muds comprise high-density dispersions of fine solids in an aqueous liquid or a hydrocarbon liquid. An exemplary drilling mud is a dispersion of clay and/or gypsum in water. The solid component of such a dispersion is termed a "weighting agent" and is designed to enhance the functional performance of the drilling fluid. Although high density fluids, such as drilling muds, perform satisfactorily in many subterranean applications, they are generally unsuitable where the hydrostatic pressure gradient of the drilling fluid is greater than the fracture or parting pressure gradient of the rock surrounding the wellbore. Under these conditions the drilling fluid undesirably parts or fractures the formation rock surrounding the wellbore enabling the drilling fluid to escape into the formation rock. Parting or fracturing the formation rock also undesirably enables fluids from the same formation being drilled or other formations in fluid communication therewith to more readily invade the wellbore. Even where the hydrostatic pressure gradient of the drilling fluid is less than the fracture or parting pressure gradient of the surrounding formation rock, the weight of the high-density drilling fluid column may, nevertheless, drive an undue volume of drilling fluid into the formation rock, particularly where the formation rock is sensitive to water invasion or where the reservoir has relatively low fluid pressure that does not effectively clean up the drilling fluid invading the reservoir, thereby inhibiting subsequent hydrocarbon production from the reservoir.
Accordingly, relatively intermediate-density or low-density compositions having corresponding intermediate or low hydrostatic pressure gradients are frequently employed as drilling fluids to obviate the above-described problems encountered with high-density, water-rich drilling fluids. Low-density drilling fluids include gasses, mists and foams. Of the gas drilling fluids, air or nitrogen is generally preferred. Mists consist of liquid particulates dispersed within a gas medium, while conventional foams consist of a gas dispersed by a foaming agent within a liquid medium. Certain gas-containing drilling fluids, termed "energized" drilling fluids, have specific utility in reservoirs having low fluid pressure, wherein the energized drilling fluid helps to clean up drilling fluids that have invaded the reservoir rock. Intermediate-density drilling fluids are typically clear aqueous liquids such as water, brines, polymer-containing brines or hydrocarbon-based drilling fluids.
Foams are oftentimes more effective than other types of low-density drilling fluids in removing materials from the drilling face to the surface of the wellbore. For example, U.S. Pat. No. 3,630,286 teaches a drilling process utilizing a specific foam composition for the removal of excess water from a wellbore being drilled wherein air is the initial drilling fluid. When water from the surrounding formation invades the wellbore, a foaming agent comprising a copolymer of acrylamide and diacetone acrylamide is injected into the wellbore and mixes with the excess water residing therein. The air circulating through the wellbore contacts the foaming agent and excess water creating a foam that is an effective drilling fluid for transporting the excess water from the wellbore to the surface wellhead.
Despite the apparent advantages of low-density foams over higher-density drilling fluids, conventional foams, nevertheless, do not always posses sufficient viscosity to adequately suspend and transport rock cuttings to the wellbore surface, thereby limiting their utility in many wellbore drilling operations. Conventional foams have also been known to perform inadequately when drilling through shales or to exhibit excessive lost circulation or relative instability under certain formation conditions. In particular, conventional foams often degrade when contacted by crude oil or when placed in formations having high downhole temperatures or in formations having brines exhibiting a high salt or hardness content.
In response to these shortcomings, additives are frequently included in conventional foams to enhance their performance as drilling fluids. For example, U.S. Pat. Nos. 4,013,568 and 4,039,459 teach the inclusion of a lignite additive to control lost circulation and an ammonium or alkali metal polyacrylate additive to increase the viscosity and cuttings-carrying ability of a conventional foam drilling fluid in high temperature applications. U.S. Pat. No. 4,088,583 teaches the inclusion of carboxyalkyl cellulose ethers and polyalkyleneoxide polymers additives to control lost circulation and stabilize a conventional foam drilling fluid in high temperature applications. U.S. Pat. No. 3,738,437 teaches a drilling process, wherein a fast-drilling, clear, low-density drilling fluid is utilized until enhanced mechanical stabilization or shale stabilization of the wellbore is required, at which time the low-density drilling fluid is converted to a high-density drilling mud. The low-density drilling fluid can be a liquid, gas, mist or foam as long as the fluid has a density and viscosity no greater than those of water. The low-density drilling fluid is converted to a drilling mud by suspending a prehydrated bentonite therein to mechanically stabilize the wellbore and further by adding a solution of partially hydrolyzed polyacrylamide and an alkali metal halide thereto to stabilize shale surrounding the wellbore.
Despite the teaching of such low-density drilling fluids in the prior art, a need remains for alternate stable and effective low-density drilling fluids. Accordingly, it is an object of the present invention to provide a drilling fluid that performs effectively in conventional drilling operations. It is a particular object of the present invention to provide a drilling fluid that effectively suspends and transports rock cuttings to the surface during a drilling operation. It is another object of the present invention to provide a drilling fluid that performs effectively in a wellbore under a broad range of subterranean conditions. It is a particular object of the present invention to provide a drilling fluid that performs effectively when drilling through a shale formation or a subterranean formation having a relatively low fracture or parting pressure gradient. It is another object of the present invention to effectively perform in a drilling operation without unduly damaging water sensitive formations. It is yet another object of the present invention to provide a drilling fluid that effectively performs as an "energized" drilling fluid to clean up drilling fluid invading the reservoir rock in reservoirs having relatively low fluid pressure. It is still another object of the present invention to provide a drilling fluid that is relatively stable under harsh formation conditions including the presence of crude oil, high temperatures, high salinity brines or high hardness brines. It is further an object of the present invention to provide such a drilling fluid that is cost effective, having practical economic utility in the field. It is still further an object of the present invention to provide a drilling process employing a drilling fluid that is chemically and operationally uncomplex. It is yet a further object of the present invention to provide a drilling fluid that is environmentally compatible and non-toxic.